Overload relays are electrical switches typically employed in industrial settings to protect electrical equipment from damage due to overheating in turn caused by excessive current flow. In a typical case, the electrical equipment is a three-phase motor which is connected to a power source through another relay commonly referred to as a contactor. A typical contactor is a heavy duty relay having three switched power paths for making and breaking each of the circuits connected to the three phase power source. The motion required to make and break the contacts is provided magnetically as a result of current flow through a coil which in turn is energized by a current whose flow is controlled by another switch, typically remotely located.
In a conventional setup, an overload relay is connected in series with the control switch for the coil of the contactor. When an overload condition is detected by the overload relay, the same cuts off power to the coil of the contactor, allowing the contactor to open and disconnect the electrical equipment that is controlled by the contactor from the source of power to prevent injury to the electrical equipment.
In the past, overload relays have utilized resistive heaters for each phase which are in heat transfer relation with a bimetallic element which in turn controls a switch. When an overload is sensed as, for example, when there is sufficient heat input from the resistive heater to the bimetallic element, the bimetallic element opens its associated switch to de-energize the contactor coil and disconnect the associated piece of electrical equipment from the source of power.
More recently, the resistive heater-bi-metallic element type of relay has been supplanted by electronic overload relays. See, for example, commonly assigned U.S. Pat. No. 5,179,495 issued Jan. 12, 1993, to Zuzuly, the entire disclosure of which is herein incorporated by reference. Outputs of such circuitry typically are relatively low powered and as a consequence, in order for the output to control the contactor coil current, a solid state switch may be required. The solid state switch may, in turn, control flow to a relatively low power contact mechanism which in turn is operable to control the flow of current to the contactor as well as to operate an indicator. In the usual case, the indicator will be a light which will be illuminated upon the occurrence of a disconnect resulting from an overload. One such contact mechanism is disclosed in my commonly assigned copending application entitled, "Trip Mechanism for an Overload Relay", Ser. No. 08/838,904, Filed Apr. 11, 1997, the entire disclosure of which is herein incorporated by reference.
The mechanism therein disclosed works extremely well for its intended purpose. However, because the same uses so-called "bridging" contacts, assembly is somewhat more difficult, increasing its cost. Moreover, bridging contacts may pose reliability problems when a circuit is to be made (as opposed to broken), particularly at low currents or loads that are associated with solid state devices. Specifically, in a bridging contact, two spaced fixed contacts are employed along with a moveable contact bar. The contact bar must make good electrical contact with both of the fixed contacts in order to complete a circuit with the consequence that if either contact is deteriorated as a result of arcing or the like, or if grime enters the switching mechanism, the circuit cannot be made. Because two contacts are involved, the likelihood of failure may be as much as doubled over the situation where only one contact is employed.
The present invention is directed to overcoming one or more of the above problems.